hardware: October 2008 Archives

The SIBYLS beamline Kohzu monochromator has often exhibited hysteresis when moving the theta 2 motor. This manifests itself in a decaying beam and suboptimal intensity. In order to fully understand the problem it helps to have a general understanding of the motors in the monochromator. The figure below illustrates the various motors and their relative motions.

The axis of the Theta motor lies on the surface of the first crystal and the angle of Theta determines the wavelength of X-rays that will be selected for a particular experiment. The second crystal is designed to take that monochromatic X-ray beam coming from the first crystal and reflect it down the beam pipe to the SAXS or MX endstation. The second crystal is ~30 meters from the crystal sample when doing a crystallography experiment and it is critical that the um-sized X-ray beam be aimed directly at the small crystalline samples. We use the Chi 2 motor on the second crystal to steer the beam from left to right (horizontal beam steering) and we use the M2 mirror to move the beam up and down (vertical beam steering). The axis of the Theta 2 motor lies on the surface of the second crystal, and it is used to align the surface of the second crystal so that it is perfectly parallel to the first crystal thus maximizing the flux of the X-rays that exits the monochromator. Typically we tuneup the beamline before a user starts their shift and during this tuneup procedure we optimize the angle of the theta 2 motor by monitoring an ion gauge positioned at the exit tube of the mono (Imono Out) and maximize the current. If we then monitor the Imono Out gauge for several hours after the tuneup procedure is complete we sometimes see that the intensity of X-rays falls precipitously.